1. Field of the Invention
It has recently been shown by Ilenda et al. in U.S. Pat. No. 4,957,974, incorporated herein by reference, that segmented copolymers, such as graft copolymers, of polyolefins, such as polypropylene, within a specific molecular weight range, and of polymethacrylates within a certain composition and molecular weight range, are useful additives for polyolefins for imparting melt strength. Such segmented copolymers are also useful for the compatibilization of polyolefins and polar polymers. An improved process for the manufacture of these segmented copolymers has been sought to lower cost of manufacture and to yield the product in a more suitable particulate form.
2. Description of the Prior Art
The term "segmented copolymer" refers to polymers wherein at least one segment of polymer A is chemically joined to at least one segment of different polymer B, and encompasses block copolymers, where the segments are joined at least one end of the segments, and also graft copolymers, where there may be a trunk of polymer A to which one least one segment of polymer B is attached at a site on the trunk which is not at the end. Because it is difficult cleanly to separate and analyze polymers where a vinyl monomer such as styrene or methyl methacrylate is polymerized in the presence of a crystalline polyolefin, such as polypropylene, and because the possibility exists for both block and graft copolymers to be formed, we have chosen to use the inclusive term "segmented copolymers."
The prior art discloses many grafted polymers from vinyl monomers onto pre-formed polyolefins. Japanese Unexamined Patent Application Kokai 03-139510 discloses block copolymers as other segmented polymers which are useful in the same manner as the grafted segmented polymers disclosed by Ilenda et al.
The prior art further discloses many technologies to prepare segmented copolymers, especially by the polymerization of vinyl monomers in the presence of pre-formed polyolefins, such as in solution, in emulsion, in a solvent-swollen aqueous dispersion, and in an aqueous dispersion without a solvent. Again, relatively few of these references relate to aqueous-related processes wherein the vinyl monomer is an ester of a lower alkyl methacrylate, and none disclose the specific polymers with high molecular weight grafted chains as taught by Ilenda et al.
The methods disclosed for the polymerization of methacrylate ester monomers, such as methyl methacrylate, in the presence of a crystalline polymer, such as polypropylene, utilize methods which require a period of contact between the polymer and the monomer to be polymerized, which may further require the use of a solvent. The art, as exemplified by Grigo et al., U.S. Pat. Nos. 4,370,450 and Yui et al., 4,097,554, does not teach a rapid method for conducting the polymerization, and such a rapid method is desirable for commercial production.
A major difficulty with such rapid methods is achieving penetration of the crystalline polymer particle by the monomer, which is best effected by a co-solvent which is essentially inert to the free-radical polymerization process, and further by the use of the polyolefin in the form of flakes, pellets, and preferably porous spherical particles. A second major difficulty is the tendency of the polyolefin to clump upon contact with the monomer/solvent mixture during the initial stages of polymerization. Dispersing agents have been taught for the older process known to the art, but these are ineffective in the present process.
Thus, the art does not teach how to accomplish the desirable goal of a rapid process for forming in an aqueous medium the graft copolymer of the composition discovered by Ilenda et al., and to maintain that graft polymer in particulate form during polymerization, solvent removal, and final isolation.